When I tried to logged into the Magento Connect Manager with my Magento admin ID and password, I am unable to log in and come out with the message "Access Denied". As I am the only administrator of the store, how should I grant the access right to myself? Thanks
and click Administrators role.
Check your Role Resources settings just in case, Resource Access dropdown should be already set to All for administrators.
Without changing anything just click “Save Role” button, so that Magento re-saves all permissions.
You should be able to access your new extension now without problems.
Try this LINK it ha a lot of info on this point
I had this same problem. There may be corruption in your magento connect config file.
To fix it, delete or rename the file "[install path]/downloader/connect.cfg"
The file will be recreated after you successfully log in again.